Refineries use large numbers of fin-fan systems to maintain cooling temperatures within various refining processes. In a typical installation, the fin-fan systems operate intermittently to automatically control the cooling air which flows over banks of cooling coils containing hot oil. A typical fin-fan unit is usually 30 feet above the ground and have fan blades ranging from 6' to 14' in diameter.
In the past, the fans of these systems have been driven by v-belts, with 15-60 horsepower motors. As a fin-fan system is typically subjected to the high temperatures, up to 250.degree. C., generated by the particular refining process, the high temperatures result in significant slippage of the v-belt due to a high internal heat. This may result in lengthening of the v-belt which leads to the need for regular re-tensioning and increased frictional wear of the belt. The high tensioning of the belt also places abnormally high loads on both the motor shaft, the fin shaft and all bearings as well as causing the belts to elongate even more. This ultimately may require premature bearing, motor and belt replacement. The failure of belts, motors or bearings may also result in inadequate cooling of the associated process leading to costly down-time of the cooling equipment.
The replacement of belts in a fin-fan is a major job, both in view of the ambient temperatures of the area surrounding the fin-fan and the confined area where the belt is located. Furthermore, in refinery processes that are operating 24 hours a day, breakage may occur at any time, in particular when maintenance staff are off-duty which again may lead to costly down-time.
Accordingly, there has been a need for a fin-fan device that reduces the maintenance requirements for the individual fin-fan units.
In response, the industry has moved away from v-belts in favour of timing belts which offer the advantage of mating cogged surfaces to provide positive engagement between the belt and the drive sprocket. This system, however, has created additional problems which the present invention also solves.
In both v-belt and timing belt systems, fin-fans will windmill backwards when not being used as external wind and air currents move over the exposed blades. When the fin-fan is subsequently turned on, high forces are applied to the shaft, belt, bearing and electric motor to overcome the backward momentum of the entire fin-fan system. With a v-belt system, the effect of the backward momentum of the shaft may be lessened by slippage of the belt. In a v-belt system, at the moment of start-up, the belt may start to slip and effectively slow the backward momentum of the shaft and provide a smooth transition to forward rotation. However, the effect of a slipping belt greatly increases the frictional wear on the belt leading to premature replacement of the belt.
With the use of a timing belt, there is no opportunity to slow the backward movement of the shaft at the moment the motor is turned on by belt slippage as there is with a v-belt. Instead, turning the motor on results in a high current load to the motor as the motor attempts to slow and reverse the direction of the shaft in a very short period of time. The sudden torque load to the shaft may also result in breakage of the belt as the belt is subjected to high tension forces. Accordingly, as in the v-belt system, the repeating on/off cycle of a fin-fan system leads to premature motor, bearing and/or belt failure.
In addition to the direct and indirect expense of replacing belts, motors and/or bearings, the task is dangerous to undertake in view of the high temperatures and the potential for uncontrolled moving parts in the work area.
In particular, replacement of an old or broken timing belt is a problem in that the shaft may be freely rotating when the belt is being replaced. Safety considerations require that the shaft is held stationary when a new belt is mounted and tensioned on the fin-fan drive sprockets. If, for example, it is a windy day when a belt breaks or needs to be replaced, it may be extremely dangerous for a worker to enter the fin-fan area for the purposes of performing maintenance.
Accordingly, there has been a need for an anti-rotation device for fin-fans that addresses the above problems in order to reduce mechanical stress and, hence, maintenance on the fin-fan system. As well, there has been a need for an anti-rotation device that improves safety and reduces the risk to operators and maintenance personnel. In particular, there has been a need for an anti-rotation device for fin-fans that prevents backward rotation of the fin-fan when the fin-fan is non-operational. Furthermore, there has been a need for a device that is relatively simple to install on existing fin-fan systems. There has also been a need for an anti-rotation device that compensates for vibration or runout in a rotating vertical shaft to improve the reliability of the anti-rotation device. Further still, there has been a need for an anti-rotation device that prevents rotation of the fin-fan shaft during maintenance operations.